Use let instead of const

Fixes #97

Author: ekilah

src/helpers/createExports.ts

@@ -39,7 +39,7 @@ export const createExports = ({
     .map(classNameToNamedExport);
 
   let dts = `\
-declare const classes: {
+declare let classes: {
 ${processedClasses.map(classNameToProperty).join('\n  ')}
 };
 export default classes;